Superjunction devices employ a structure composed of alternately arranged N-type pillars and P-type pillars. When an N-channel superjunction metal-oxide semiconductor field-effect transistor (MOSFET) is in an on-state, the on-state current will flow in the N-type pillars; while when it is in an off-state, the depletions between N-type and P-type pillars will enable a high breakdown voltage. Therefore, the breakdown voltage will not be decreased even when a relatively thin N-type epitaxial layer with a high concentration of N-type dopant is formed. Thus, the N-channel superjunction MOSFET can have a low on-resistance (Rson) while maintaining a high breakdown voltage. Similarly, a P-channel superjunction MOSFET also has such property while having the conductivity types of relevant elements opposite to those of the N-channel one.
Despite their merit of having a high breakdown voltage and a low Rson at the same time, there are still many issues that need to be addressed for conventional superjunction devices, such as the difficulties in forming the P-type pillars and N-type pillars and designing a proper termination structure.
As superjunction devices have a higher dopant concentration in their epitaxial layer, the termination design with floating rings and field plates that has been adopted in vertical double-diffused metal-oxide semiconductor (VDMOS) transistors is not applicable to conventional superjunction devices. Instead, conventional superjunction devices typically employ a termination structure in which the trenches are designed to have an annular shape. Such termination structure is achieved by first forming the annular-shaped trenches and then filling the trenches with an epitaxy material such as silicon to form P-type pillars or N-type pillars therein. In this process, the step of filling the annular-shaped trenches has become a great challenge to existing silicon epitaxial growth technologies. This is because the silicon epitaxial growth rate is related with the crystal orientation of the silicon surface, and epitaxial growth rate varies, and hence the filling quality varies when applied to surfaces with different crystal orientations. For the aforementioned termination structure, as the crystal orientations around the corners of an annular-shaped trench are keeping changing, the epitaxial growth rate of a trench filling process around the corners will be varying accordingly, and therefore, the filling of silicon in these portions is most challenging and defects (e.g., voids) are generally left there after the silicon filling process is completed. FIG. 1 is a top view schematically illustrating a corner portion of a conventional superjunction device. Referring to FIG. 1, parallel and equally-spaced trenches 101 are formed in a current-flowing area (i.e., an active area). A termination structure includes multiple annular-shaped trenches (only one of them is shown in FIG. 1), each of which is a corner-rounded rectangle enclosing the current-flowing area. Additionally, each annular-shaped trench is composed of a pair of first sides 102 which are straight and parallel to the trenches 101, a pair of second sides (not shown) which are straight and perpendicular to the trenches 101, and four arc-shaped portions 103 each of which is positioned at a corner of the annular-shaped trench for connecting a first side 102 to a second side that is perpendicular to the first side 102. As shown in FIG. 1, as the annular trench is always changing its extension direction in the arc-shaped portions 103, the Miller indices of trench sidewalls are varying accordingly and the Miller indices of the arc-shaped portions do not belong to the same family of crystal planes with that of the first or second straight sides. As in a silicon filling process the silicon epitaxial growth rate is dependent on the Miller indices of trench sidewalls and trench bottom face, the growth rate in the arc-shaped portions 103 is position-dependent and different with the growth rate in the trenches 101 and straight sides of the annular-shaped trenches, thus making the arc-shaped portions 103 difficult to be properly filled. FIG. 2 is an image showing the silicon filling result of arc-shaped portions of a conventional superjunction device. As indicated by a dashed circle in the figure, voids which will deteriorate the performance of the device are present in the arc-shaped portions 103 of the termination structure.